Gas-liquid-solid catalytic reactors, also called three-phase reactors, are widely used in various industries, such as refining, chemical, and petrochemical industries, to carry out various reactions. In refining gas liquid three phase reactors are used for hydroprocessing of hydrocarbon feedstock. Hydrocarbon feedstock is generally available as crude oil, which includes various petroleum fractions like gasoline, kerosene, diesel, wax, and heavy oils, along with unsaturated hydrocarbon compounds, such as olefins and aromatics, and hetero atom impurities, such as sulphur, nitrogen, etc. The hydrocarbon feedstock has to be hydroprocessed to a suitable quality for production of various petroleum products. For example, long chain hydrocarbons in heavy oils have to be broken down into smaller chain hydrocarbons, the impurities have to be removed and the unsaturated compounds have to be saturated. Such reactions are carried out using the three-phase reactors.
These reactors can be operated in co-current or counter-current mode. If the reactor is designed in such a way that the reactants flow in the same direction within the reactor, it is termed as a co-current three-phase reactor. When the direction of flow of reactants is opposite to each other in the reactor, it is known as counter-current three-phase reactor.
The three-phase packed bed reactors, also referred to as trickle bed reactors, are commonly used for hydroprocessing. Hydroprocessing is a highly exothermic process involving treatment of hydrocarbon feedstock with hydrogen in the presence of a suitable catalyst to achieve various objectives like desulphurization, denitrogenation, hydrogenation, hydrocracking, and isomerization for production of fuels and lubes of desired quality. It is generally carried out in adiabatic mode with intermediate quenching to check the rise in temperature due to the exothermic nature of the reactions. In hydroprocessing, hydrogen react with hetero atoms to produce gases like hydrogen sulphide and ammonia, thereby removing the hetero atoms from the hydrocarbon feedstock. Further, the treatment with hydrogen also results in hydrogenation of unsaturated hydrocarbon molecules and hydrocracking of long chain molecules.
Conventionally, hydroprocessing of hydrocarbon feedstock is done in three-phase packed bed reactor with co-current down flow mode of operation at elevated pressure and temperature. In the conventional co-current down flow hydroprocessing, high gas to oil ratio is used, and thus, gas phase is continuous phase and liquid is dispersed. Consequently, it leads to undesired vaporization of hydrocarbon feed during hydroprocessing of hydrocarbon feedstock. In addition, higher gas/oil ratio leads to increased pressure drop across the reactor, and thus, a high pressure differential within the reactor. Further, the co-current reactors have a high mean flow path for the reactant gas, which leads to additional pressure drop along the length of the reactor and significant difference in pressure of the reactor at the entrance and exit.
On the other hand, as the hydroprocessing reactions proceed within the reactor, more and more of hydrogen sulphide (H2S) and ammonia (NH3) are generated. As a result, there is increase in the concentration of H2S and NH3, and thus, the partial pressure of hydrogen is reduced. The reduced partial pressure of hydrogen leads to reduced rate of desulphurization, denitrogenation, and saturation reactions in the reactor. This often leads to high severity operation to meet product quality specifications, in turn causing nonselective cracking of hydrocarbon feedstock to light ends and shorter run lengths due to increased catalyst deactivation rates.
Further, gas phase holdup increases continuously from reactor entrance to exit as gas phase hydroprocessing products are generated. This results in inefficient utilization and insufficient wetting of catalyst by liquid phase reactant and higher catalyst requirement for given through put and desired product quality and yields. In addition, these units are energy intensive and part of this energy is unnecessarily utilized for vaporization of feed as reactant gas and feed are heated together in reactor feed furnace.
On the other hand, in counter-current packed bed reactors, the reactors are designed in such a way that the gas and liquid reactants are introduced from vertically opposite ends of the reactor into the reactor column and flow in opposite directions. Counter current reactors facilitate ultra-low sulphur levels to be achieved efficiently while treating hydrocarbon feedstock since, during operation; a major part of the middle region is in an H2S lean environment. However, these reactors also have high differential pressure and a limited range of gas-liquid flow rates under which they can operate without flooding. Flooding typically occurs when the flow rate and pressure of the gas phase is high enough to prevent the downward flow of the liquid phase. Due to flooding, the gas tends to lift the liquid out of bed and there is poor contact between the two phases. As a result, the reactors have to be operated at lower flow rates, which make them commercially unviable. Counter-current hydroprocessing has not achieved commercial acceptability as the drawbacks associated outweigh the benefits.
Further, in both co-current and counter-current reactors, dry spots are created in the middle region during operation due to uneven distribution of feed, feed vaporization, and higher gas holdup. Dry spot is a region in the middle region where the catalyst is devoid of liquid hydrocarbon feed and so cannot participate in the reaction. The occurrence of dry spots leads to under utilization of catalyst in the reactors, which can be significant on an industrial scale. Thus, in both counter-current and co-current reactors, a part of the hydrocarbon feed is either wasted or incompletely processed. To ensure the required quality of treated hydrocarbon, the output stream has to be hydroprocessed again in multiple stages to overcome thermodynamic equilibrium, which increases the cost of operation.
To overcome some of these difficulties, state-of-the-art trickle bed reactors are being designed with inter bed separators for removal of inhibitory compounds, multi stage operation with temperature control and catalysts to overcome equilibrium limitations and maximizing intermediates, efficient gas liquid distributors and internals for improved gas liquid distribution and catalyst wetting. In similar lines, hybrid contacting pattern along with inter-stage separators were described in prior art to address inhibition effects during desulfurization and separation of light ends formed during cracking before passing feed to next stage/reactor.
A third type of reactor, called radial flow or cross flow reactor has been proposed in the art to overcome some of the disadvantages of the counter-current and co-current reactors. In the radial reactor, the liquid and the gas streams flow in radial direction. Radial flow reactors are commercially used for applications such as naphtha reforming process, ammonia synthesis, ethyl benzene dehydrogenation etc. While theoretically such reactors have reduced pressure drops, the commercial use of such reactors is limited to high throughput application and low pressure drop requirements.
Recent technologies advocate liquid phase hydroprocessing wherein the liquid is pre-saturated with hydrogen and hydroprocessing is carried out in absence of gas phase in the reactor. The advantages suggested are near isothermal conditions in the reactor and avoiding gas compression and gas recycle. However, as the solubility of hydrogen is limited good amount of product needs to be recycled for carrying the soluble hydrogen. The liquid recycle requirement for hydrogen intensive applications such as cracking and treating of high sulfur feedstocks is expected to be still higher. Further the inhibitory effects of NH3 and H2S are expected to be similar to conventional trickle bed reactors.
Therefore it is desirable to have improved multiphase reactor design which overcomes shortcomings of prior art and enables low severity operation, minimizes feed vaporization and non selective cracking, enhances reaction rates for desired reactions, minimizes reactor pressure drop and reactant gas consumption. Thus, there remains a considerable need for apparatus and methods for efficient hydroprocessing of hydrocarbons.